The present invention relates to a method for calculating a tolerable value for fluctuations in power supply voltage and a method of testing. More specifically, the present invention relates to a method for calculating a tolerable value for simultaneous switching noise in an input/output circuit having a differential input, and a method for testing a semiconductor integrated circuit using the calculated tolerable value.
Due to the miniaturization of recent semiconductors, it has become significant that a measure for preventing power supply noise be designed when designing a semiconductor integrated circuit such as a system LSI. The tolerable value for fluctuations in power supply voltage of an input/output circuit must be obtained to estimate the noise.
Semiconductor integrated circuits have become highly integrated due to the miniaturization of semiconductors. This has increased the number of input/output circuits of the semiconductor integrated circuit. Voltage fluctuation occurs in power supply wires when a plurality of input/output circuits are switched at the same time and at the same phase in a semiconductor integrated circuit. The fluctuations in the power supply voltage that occurs in this manner is referred to as simultaneous switching noise (SSN) (see e.g., R. Senthinathan and J. L. Prince, “Simultaneous Switching Noise of CMOS Devices and Systems”, Kluwer Academic Publishers, 1994, pp. 23-28). The simultaneous switching noise must be estimated and reduced since power supply voltage fluctuations may lead to erroneous functioning of the semiconductor integrated circuit. Therefore, various methods for estimating simultaneous switching noise (see e.g., Japanese Laid-Open Patent Publication No. 2005-38400) and for reducing simultaneous switching noise have been proposed.
When designing a semiconductor integrated circuit, a designer must determine whether an input/output circuit would actually function erroneously due to simultaneous switching noise. More specifically, the designer performs calculation and computer simulations to obtain the tolerable value for power supply voltage fluctuations in an input circuit and simulate simultaneous switching noise. Then, the designer compares the calculated tolerable value with a noise value to determine whether or not the noise value is tolerable.
In the prior art, voltage values of a high voltage power supply (external power supply) and a low voltage power supply (ground) of the input circuit are changed to determine the voltage value of when the output signal of the input circuit changes as the tolerable value for voltage fluctuations in the external power supply and the ground. For example, referring to FIG. 1(a), a variable power supply 12 is connected between a low voltage power supply terminal of an input circuit 11 and ground. In this case, when an external input terminal of the input circuit 11 is provided with a signal St having an H level and the voltage (ground voltage) supplied to the low voltage power supply terminal changes, an output signal of the input circuit 11 is observed at a predetermined observation point (point marked by x in the drawing). The voltage at the observation point changes from an H level to an L level as the ground voltage increases, as shown in FIG. 1(b). The ground voltage of when the voltage at the observation point changes from an H level to an L level is determined as the tolerable value for ground voltage fluctuations.
In the same manner, referring to FIG. 2(a), the variable power supply 12 is connected between a high voltage power supply terminal of the input circuit 11 and the external power supply. In this case, when an external input terminal of the input circuit 11 is provided with a signal St having an L level, the voltage (external power supply voltage) supplied to the high voltage power supply terminal changes, and the voltage at the output terminal of the input circuit 11 is observed. The voltage at the observation point changes from an L level to an H level as the external power supply voltage decreases, as shown in FIG. 2(b). The external power supply voltage of when the voltage at the observation point changes from an L level to an H level is determined as the tolerable value for voltage fluctuations in the external power supply.